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Probably a capitellid. These are primarily sediment dwellers and feed by selectively choosing organic particles or by swallowing sand grains & digesting the attached organics. They don't have jaws; instead they have an eversible sac called the proboscis which they used to engulf particles. the proboscis can also be used to help them move through the sediment. They expand the sac & use that as an anchor to pull themselves through the sand.

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Cheers, Leslie
So many worms, so little time...
Natural History Museum of Los Angeles County

I'm pretty sure [as I suspect you are] that it's not your ordinary bristleworm. Doesn't seem to be a eunicid. Leslie H may well spot this thread if it stays up long enough. She'd have a fair idea. Is any part of the body thicker than the other---[peanut worm]? Doesn't look to be. It's sure not a spaghetti worm. I'd house this baby in the sump until ID'ed.

EDIT: ha! and there is the redoubtable Leslie H! Capitellid. New one on me.

Capitellid thread worms are one of the first groups of animals to re-colonize an area impacted by dredge spoil dumping or by an oil spill. They are particularly known for their high tolerance for organic pollution.

o.c.d. - most polychaetes have the same general body structure. Distinctions come from the appendages and type of setae (= bristles). The books usually list 10 or less of the bigger worms. There's perhaps 10,000 described species of polychaetes and a unknown - but high - number of undescribed ones. The same is true for shrimp, crabs, and all the other inverts. The incredible biodiversity is part of what makes marine biology so much fun.

__________________
Cheers, Leslie
So many worms, so little time...
Natural History Museum of Los Angeles County

Chuck's Addiction is the best photo site for reefers - http://home2.pacific.net.ph/~sweetyummy42/index.html He lives in the Philippines where he can pick up a constant supply of live rock and critters directly from the reefs. There are various types of worm databases but not one that combines pictures and life history information for a lot of species. If you search for the genera names Capitella, Heteromastus, Mediomastus, and Notomastus (there are more than that) you'll find images. If you do a search for a genus name & another term like "food" "diet" "life history" "feeding" "behavior" you'll find out more about them.

__________________
Cheers, Leslie
So many worms, so little time...
Natural History Museum of Los Angeles County

Agu -- capitellids & cirratulids (the hair worms) are so well known for their ability to population low oxygen & pollution areas that they are considered prime pollution indicator species. A bottom sample from a clean area will have many different species. A sample from an outfall or impacted area will have only a few species but these will occur by the thousands or tens of thousands, and caps & cirris will be the most abundant. Many labs keep capitellids on hand for toxicity studies so there should be a lot of information available on how to culture them.

__________________
Cheers, Leslie
So many worms, so little time...
Natural History Museum of Los Angeles County